The Properties of the Magnetic Shape Memory Alloy Actuator

Components made of alloys with a magnetic shape memory (MSMA) are not commonly used in industrial systems or prototype solutions. The reasons behind this include the cost and implementation difficulties attached to MSMA alloys synthesis technology. The article presents the results of a prototype actuator in which the active element is a rod made of Ni-Mn-Ga alloy and with dimensions of 2x1x20 mm. INTRODUCTION Shape memory alloys (SMA), which constitute a group of smart materials that change their crystallographic structure under the influence of temperature [1], have been the subject of study for several decades now. Recently, considerable attention has been focussed on alloys with a magnetic shape memory (MSMA), [7]. The technical application potential of these alloys arises from their ability to deform in a range of 3 to 8%, at frequencies of up to 1 kHz. The first experimental actuators, in which the active element is an MSMA alloy, have already been created. The literature contains no information about the commercial applications of these alloys. Their drawbacks are low compressive stress limits, at which the shape memory effect persists, and the need to produce a magnetic field with an induction of 1 to 2 T [6]. This paper presents the results arising from the characteristics of one of the above-mentioned experimental actuators, the operating principle of which is based on the phenomenon of magnetic shape memory (MSM). The actuator generates movement as a result of changes in the linear dimensions of the MSMA material, with dimensions of 20x2x1 mm and made of Ni-Mn-Ga alloy subjected to further surface treatment. Although such treatment reduces the maximum deformation of the MSMA element, it improves its mechanical properties in comparison with a ‘pure’ alloy [3]. 2. MSMA ALLOY OPERATING PRINCIPLE The operating principle of the MSMA alloy is explained by the example of the most common Ni-Mn-Ga alloy. In a high-temperature state, that alloy remains in the non-deformed phase of the austenitic structure, which is characterised by the regular shape of an elementary cell. The shape of the elementary cell in the high-temperature phase is shown in Fig. 1a, while the tetragonal structure of the cells in the low-temperature phase, which is the martensitic structure for option 1, is given in Fig. 1b. During cooling, the sample must be subjected to constant compressive stress σxx in direction [100], subject to the condition: σsv < σxx< σb (σsv: compressive stress conditioning the tetragonal structure of the martensite formation in Option 1, the sides of the elementary cell a > c; Fig . 1b, σb – blocking stress). y, [010] x, [100] z, [001]